However when I stopped the brake pedal went to the floor

I recently bought this car from a small used car lot, no information on previous owner. Took a trip approx 150 miles away from my home, no problems on the way there. The van was parked all day at Wild Adventures. That night we did have some traffic to set through getting out. When we got to the interstate and I tried to increase my speed over 40 mph, I felt like I was having a flat tire. Stopped and checked, the front brakes were hot. Got back on the road since it was not far to our destination for the night, only about 10 miles. Once I increased my speed over 60 this leveled out. However when I stopped the brake pedal went to the floor why?